Hard disk drives (HDDs) are widely used for high-density information storage. HDDs are commonly found in computer systems traditionally associated with this type of storage, such as servers and desktop computers. However, HDDs having smaller form factors, such as one-inch drives, can also be found in hand-held electronic devices, such as music players and digital cameras.
Higher storage capacity in HDDs can be achieved by increasing storage density. Storage density is currently doubling roughly every year and the highest storage density presently achievable using conventional technology, such as by recording data in bit cells which are arranged longitudinally in the magnetic recording medium and reading data using so-called “spin value” read heads, is about 100 Gb/in2.
However, as storage density in HDDs continues to increase, then recording media and read heads encounter the problem of the superparamagnetic effect.
The superparamagnetic effect arises when the size of a ferromagnetic grain is sufficiently reduced that the energy required to change direction of magnetisation of the grain is comparable to the thermal energy. Thus, the magnetisation of the grain is liable to fluctuate and so lead to data corruption.
For recording media, a solution to the problem has been demonstrated which involves arranging bit cells perpendicularly (rather than longitudinally) to the surface of the recording medium which allows each bit cell to be large enough to avoid the superparamagnetic effect.
To address this problem in read heads, it been proposed to avoid using any ferromagnetic material and to take advantage of the so-called extraordinary magnetoresistance (EMR) effect.
A device exhibiting the EMR effect is described in “Enhanced Room-Temperature Geometric Magnetoresistance in Inhomogeneous Narrow-Gap Semiconductors”, by S. A. Solin, T. Thio, D. R. Hines and J. J. Heremans, Science volume 289, p. 1530 (2000). The device is arranged in a van der Pauw configuration and includes a highly conductive gold inhomogeneity concentrically embedded in a disk of non-magnetic indium antimonide (InSb). At zero applied magnetic field (H=0), current flows through the gold inhomogeneity. However, at non-zero applied magnetic field (H≠0), current is deflected perpendicularly to the field-line distribution, around the gold inhomogeneity and through the annulus. This gives rise to a drop in conductance.
Currently, high mobility narrow gap semiconductors with low carrier density, such as indium antimonide (μn=7×104 cm2V−1s−1 at 300° K), indium arsenide (μn=3×104 cm2V−1s−1 at 300° K) and gallium arsenide (μn=8.5×103 cm2V−1s−1 at 300° K), seem to be the best candidates for EMR-based read heads.
“Nanoscopic magnetic field sensor based on extraordinary magnetoresistance” by S. A. Solin, D. R. Hines, A. C. H. Rowe, J. S. Tsai, and Yu A. Pashkin, Journal of Vacuum Science and Technology, volume B21, p. 3002 (2003) describes a device having a Hall bar-type arrangement having an indium antimonide/indium aluminium antimonide (InSb/In1-xAlxSb) quantum well heterostructure.
A drawback of this device is that it requires a thick (i.e. about 75 nm) passivation layer to protect and confine the active layer as well as an insulating coat in the form of a layer of silicon nitride. This increases the separation between the channel and magnetic media and so reduces magnetic field strength and, thus, the output signal.
Silicon does not require passivation and silicon-based magnetic field sensors exhibiting magnetoresistance are known.
For example, EP-A-1 868 254 describes a device exhibiting the extraordinary magnetoresistance effect having a channel formed of silicon. A conductor formed of titanium silicide or highly-doped silicon acts as a shunt and is connected to the channel along one side of the channel. Leads are connected to and spaced along the channel on the opposite side of the channel.
However, silicon has lower mobility and so device performance tends to be poorer.
The present invention seeks to provide an improved magnetoresistance device.